1. Field of the Invention
The present invention is related generally to steam turbines, and more particularly to crossover piping used in such steam turbines.
2. Statement of the Prior Art
"Crossover" piping is typically employed in a steam turbine-generator frame to facilitate the passage of a flow of steam from a relatively high pressure portion of the steam turbine (e.g., a high pressure turbine element or an intermediate turbine pressure element of the steam turbine) to a relatively lower pressure portion thereof (e.g., a low pressure turbine element of the steam turbine). As is readily apparent, such piping between the turbine elements of a steam turbine should not constitute a "stiff piping system", since it must of necessity allow for the expansion and contraction and other similar such movements that go along with an operating steam turbine. A sufficient amount of flexibility can typically be provided in such crossover piping, however, by using one or more hinged bellows expansion joints. That is, where space limitations and other design considerations result in configurations of insufficient flexibility, capacity for deflection within allowable stress range limits may be increased to provide a semirigid, or even a non-rigid system, and the expansion effects essentially eliminated by a free-movement system.
As is well known, expansion joints for semirigid or non-rigid systems can be restrained against longitudinal and lateral movement by the hinges with the expansion element under bending movement only, and are referred to as "rotation" or "hinged" joints. Semirigid systems are limited to one plane, while non-rigid systems require a minimum of three joints (e.g., in the configuration that is typically used in crossover piping installations) for two-dimensional expansion movement. In those situations were it becomes necessary to provide three-dimensional expansion movement, five joints must be installed.
Commercial bellows elements are usually formed of a light gage (on the order of 0.05 to 0.10 inches thick) material, and are available in stainless and other alloy steels, copper and other non-ferrous materials. Multiply bellows, bellows with external reinforcing rings, and toroidal contour bellows are also available for high pressure installations. Typical of the bellows elements that are suitable for crossover piping installations are those manufactured by Westinghouse Electric Corporation for use with their turbine building blocks 245 and 271M or 271H.
There are three basic designs of crossover piping installations that are presently used for conventional steam turbines. Two of these three basic designs (i.e., the commercial balanced expansion joint design and the link-hinge diaphragm or "LHD" design) utilize turning vanes to facilitate a flow of steam. Both designs are similar in that they employ: (1) a straight, vertical section of piping, including an expansion joint, from the exhaust of the high pressure or intermediate pressure turbine element of the steam turbine; (2) a straight, vertical section of piping into the inlet of the low pressure turbine element of the steam turbine; and, (3) a straight, horizontal section of piping, including a pair of expansion joints, which joins the straight, vertical sections. The turning vanes are used within the elbow junctions of the piping sections, and the two designs vary only in their use of a different type of expansion joint. However, the LHD design is the more popular design (especially at installations having a single LP turbine element) because it is considerably less expensive than the commercial balanced expansion joint design.
The remaining basic design of crossover piping installation that is presently used for conventional steam turbines is the "short radius" crossover pipe, which also uses the link-hinge diaphragm as expansion joints but does not use turning vanes. Such elimination of the turning vanes has the advantages of reducing the costs and complexity of the crossover piping design, but it also has the disadvantage of a poor flow of steam and attendant losses of efficient heat. That is, because of the "short radius" nature of the elbows that are used in this crossover piping design, the steam will not flow as readily as it would in through an elbow having a longer radius of turn. By definition, therefore, "long radius" crossover piping would employ turning subsections which redirect the flow of steam by substantially 90.degree. (e.g., from a vertical direction to a horizontal direction or vice versa) over a greater distance than "short radius" crossover piping.
Since bellows elements that are used in crossover piping are ordinarily rated for strain ranges which involve repetitive yielding, predictable performance is assured only by adequate fabrication controls and knowledge of the potential fatigue performance of each design. The attendant cold work on bellows elements can affect their corrosion resistance and promote a greater susceptibility to corrosion fatigue or stress corrosion. For example, expansion joints in a horizontal position (e.g., those which are used in the straight, vertical section of piping normally exiting the exhaust of the HP/IP turbine element of a steam turbine) cannot be drained and have frequently undergone pitting or cracking due to the presence of condensate whether during operations of the steam turbine or offstream.